Here's a recent interview with Dr. Franklin Chang-Diaz in which, among other things, he discusses the need for the development of small nuclear fission reactors for use in space. It's one of the better discussions of the VASIMR concept.

Quote:

What we really need is nuclear power to generate electricity in space. If we don’t develop it, we might as well quit, because we’re not going to go very far. Nuclear power is central to any robust and realistic human exploration of space. People don’t really talk about this at NASA. Everybody is still avoiding facing this because of widespread [irrational] anti-nuclear sentiment.

This cannot be emphasized enough. Power has always been a major (in many cases the major) bottleneck in space mission planning. Solar energy is cumbersome in earth orbit and useless in the outer solar system. RTGs are great, but they still don't provide nearly enough energy density for high bandwidth data transmission and other operations necessary for serious exploration. I firmly believe that the 2005 defunding of Project Prometheus was (one of) the largest long term setbacks to both manned and unmanned space exploration in recent years.

The VASIMR concept has the promise of solving the Propulsion problem of long distance space travel in the near to medium term. (Need to get to LEO first though.) It remains to be seen, however, when the Power problem will be solved. Hyperion maybe?

200kW is ridiculously small in the scale of rockets, by the way. Our very small 800lbf rocket flows 200kW through the chamber wall into the fuel before burning it. That's a very small portion of the total power output.

Notice that in none of the VASMIR stuff have they ever actually said that they produced useful thrust at a useful Isp, just that they've managed to dump 200kW into a plasma.

They have not yet talked about any actual thrust and ISP numbers, because it has not yet been tested as far as I know. The only numbers that are being "thrown" around are potential numbers. One of the mayor unknown factors is the question if the plasma gets detached from the engine at all.

you can use any element to fuel vasimr as far as i'm aware, but hydrogen would be a very bad choice in all likelihood because it's so light. i could be wrong on that, it is easy to ionize, but something heavier like argon is definitely better. i doubt the mass benefit of not using argon (really not a big concern, they need extra batteries and stuff even to run the damn thing) is worth it.

Actually H2 is the BEST choice. I was also under the false assumption that heavier was better until recently. The lighter the ejected propellant, the faster it accelerates. Since the total energy is M*V*V the velocity is all important. 2(H2)+O2 (shuttle main engines) are the best you can get to with a chemical reaction - an ISP of ~450. VASIMR's ISP has the potential to be much better than that, but a nuclear motor is really the best option.

"We can lick gravity, but the paperwork is overwhelming" -- Werner Von Braun"It's all fun and games until the potato chips get loose." (said of the ISS by Gizmodo on space.com)"A journey of a thousand miles begins with a single step. A journey of a hundred thousand miles begins with lots of flames, noise and smoke!" -- Emory Stagmer

Actually H2 is the BEST choice. I was also under the false assumption that heavier was better until recently. The lighter the ejected propellant, the faster it accelerates. Since the total energy is M*V*V the velocity is all important. 2(H2)+O2 (shuttle main engines) are the best you can get to with a chemical reaction - an ISP of ~450. VASIMR's ISP has the potential to be much better than that, but a nuclear motor is really the best option.

I just thought of something kinda off the wall. If a VASIMR engine is ejecting protons/hydrogen ions, won't the spacecraft become negatively charged due to this? And wouldn't a high powered electron gun, a beefed up version of those found in a CRT, give an incredible ISP due to the relatively tiny mass of electrons? It would also negate the problem of building up a negative charge on the spacecraft, which could be a big problem when docking.

I'm sure someone at Ad Astra has already thought of and fixed these issues though.

I just thought of something kinda off the wall. If a VASIMR engine is ejecting protons/hydrogen ions, won't the spacecraft become negatively charged due to this? And wouldn't a high powered electron gun, a beefed up version of those found in a CRT, give an incredible ISP due to the relatively tiny mass of electrons? It would also negate the problem of building up a negative charge on the spacecraft, which could be a big problem when docking

I'm not sure exactly how VASIMR solves this problem, but I know most EP types have something called a "neutralizer" at the very end of the nozzle that gives electrons to the outgoing stream of particles. This is necessary for more reasons than just keeping the charge of the spacecraft neutral (actually not really all that required - moon dust generally carries a charge of up to 10V for example due to the solar wind). More importantly, it will keep the propellant actually travelling in a straight line and make its behavior more predictable, as well as guaranteeing the particles won't do undesired things like hit the solar panels or corrode any other component they happen to come across faster than they would otherwise.

You have to neutralize your exhaust if you want to go anywhere. A positively charged exhaust and a negatively charged vehicle will attract. The exhaust will just come flying back pulling the vehicle backwards. Some will escape sure, but it is an efficiency drain. I imagine that it would be best to get a very small net positive charge on your vehicle and maintain it.